Many ground or mobile antennas transmit an elliptical beam towards a remote receiver such as, without limitation, a satellite or unmanned aerial vehicle (UAV). A direct line from the antenna to the remote receiver is the true line-of-sight from the antenna to the remote receiver. The pointing direction of the antenna is the antenna line-of-sight. The peak signal power of the elliptical beam will point in the direction of the antenna line-of-sight, but the antenna line-of-sight may or may not be pointed on the true line-of-sight, and may be off a few degrees. Here, whatever direction an antenna is pointing in before a process starts is referred to as the initial pointing direction, which may or may not be the true line-of-sight to the remote receiver.
Step track is a widely-used technique which allows an antenna to be pointed accurately at a remote receiver, so that antenna line-of-sight and hence the peak signal power of the elliptical beam is closely aligned with the true line-of-sight from the antenna to the remote receiver. A received signal strength indicator is returned from the remote receiver to indicate the strength of the signal power of the elliptical beam. Step track works by measuring the relative strength of the received signal when the antenna is deliberately mispointed by a small amount away from the initial pointing direction in two orthogonal planes. It is possible, by utilization of equations which represent curve fitting to the antenna beam shape in these two orthogonal planes, to estimate the direction of the signal peak relative to the remote receiver, and repoint the antenna toward the remote receiver.
For a transmit/receive antenna system using a common antenna for both transmit and receive, this process will, in addition to more accurately boresighting the transmit beam, automatically also boresight the receive beam. For a transmit/receive antenna system using separate transmit and receive antennas, the receive beam can be more accurately boresighted by “slaving” the receive antenna pointing to the pointing direction of the transmit beam.
This technique is almost invariably applied in the antenna azimuth and elevation planes, i.e., the antenna pointing direction is offset to either side of the initial pointing direction separately in the azimuth and elevation planes, and works well for the great majority of antenna types. There is, however, a class of antennas for which this technique does not work well, and for which, if the step track algorithm is run in the azimuth and elevation planes, will result in errors in the pointing solution derived from the step track process. The class of antennas for which the step track algorithm just described does not work well has two characteristics: the shape of the main beam of the antenna pattern is elliptical rather than circular, and the major and minor axes of the main beam profile ellipse are not generally aligned with the antenna azimuth and elevation planes.
One example of the latter is a smooth-walled conical horn antenna, possibly with an aperture-located phase-correcting lens to reduce the horn flare length. Such a horn normally supports the dominant TE11 waveguide mode, and exhibits the same field distribution in the horn aperture. The impact of this is that the main beam of such antennas is elliptical, with the H-plane 3 dB beamwidth approximately 25% greater than the E-plane beamwidth. Also as the antenna polarization is adjusted to match the polarization orientation of the outgoing wave (the beam rotates about its axis with the plane of polarization, the main beam ellipse will rotate with the plane of polarization, and will not in general align with the antenna azimuth and elevation axes).
These effects are shown graphically in FIG. 1. Polarization is adjusted so that the E-plane 110 is at an angle (the “polarization angle”) of 30° to the local vertical. Circle 108 is the elliptical −3 dB contour of the main beam, and lines 102 and 104 are the minor and major axes of the beam ellipse which align with the E-plane 110 and H-plane 112, respectively. As stated earlier, as the antenna polarization is adjusted in real time to match the polarization of the outgoing wave, which will be the case if one or both ends of the communications link are moving, the main beam ellipse will rotate with the polarization.
The deficiencies of the conventional step track algorithm, when applied to the rotated elliptical main beam are shown graphically in FIG. 2. In FIG. 2, azimuth 0° and elevation 0° is defined as the signal peak of the elliptical beam 220. For this example, the initial pointing direction of the antenna (which may be any direction), is the starting point 212 or +1.0° azimuth +1.0° elevation in antenna beam coordinates. This means that the initial pointing direction of the beam signal peak is actually −1.0° azimuth −1.0° elevation relative to the starting point 212. For the conventional step track algorithm, the four offset points 204/206/208/210, from the starting point 212 (Az0,El0) are the following coordinates for antenna pointing:(Az0+ΔAz, El0)(Az0−ΔAz, El0)(Az0, El0+ΔEl)(Az0, El0−ΔEl)where ΔAz and ΔEl are the angular offsets in the azimuth and elevation planes respectively (the amount offset from the starting point 212).
The conventional step track algorithm will first offset the antenna pointing in the azimuth plane to the points 206 and 208 on either side of the starting point 212 on the horizontal dashed line, and based on the relative received signal levels at the starting point 212 and the two points 206/208 on either side, will estimate the location of the beam peak in the azimuth plane (reference number 214) to the left on the horizontal dashed line. The point 214 is estimated as the peak of an inverted parabola, where points 206, 208 and 212 determine the parabola. The same process will then take place in the elevation plane, offsetting the antenna pointing to either side of the starting point 212 to the points 204 and 210 on the vertical dashed line. The points 204, 210 and 212 are used to estimate the location of the beam signal peak in the elevation plane as 216 on the vertical dashed line. For this particular case, the conventional step track algorithm incorrectly estimates the beam peak to be located at point 218 (0.12°, 0.16°); whereas the beam signal peak is actually located at point 220 (0°,0°).
The error occurs because the conventional step track algorithm does not correctly compensate for the angular offsets inherent in an elliptical beam that are not present in a circular beam. FIG. 3 shows how the magnitude of the step track elevation error 302 and azimuth error 304 are linearly related to the magnitude of the initial azimuth and elevation offsets to the pointing direction for peak beam power. FIG. 4 shows the dependence of the step track elevation error 404 and azimuth error 402 on the polarization angle, with an initial pointing direction (starting point) (see FIG. 2, reference number 212) at (1.0°, 1.0°). When the polarization angle is either 0° (vertical polarization), or 90° (horizontal polarization), the step track errors become zero, since for those cases the principal axes of the main beam ellipse are aligned with the antenna azimuth and elevation axes. For all other polarization angles, the step track errors are greater than zero.
An existing method of reducing the step track pointing solution errors induced by application of the conventional step track algorithm to a rotated elliptical beam is to apply the conventional algorithm iteratively. Simulation has shown that this will provide a convergent solution, however several iterations are required, and the time taken to derive an acceptably accurate pointing solution will be excessive.